Reliability, and hence fault-tolerance, are of great concern in telecommunications systems. Various strategies have been devised over time to achieve fault tolerance in telecommunications systems.
Switching fabrics have commonly used the duplication strategy, wherein two switching fabrics are provided--one active and the other standby--and the standby fabric becomes active and takes over for the active fabric when the active fabric fails. However, such duplication is very expensive and inefficient in the use of total system resources, as only one half of the resources are used at any one time.
Recently, the art has seen the advent of so-called "self-healing" network topologies, which attempt to avoid full switching fabric duplication, by adding one or more switching stages within an integrated circuit that implements the switching fabric and relying on routing algorithms to find and create new paths through the network that bypass failed switching nodes. However, these topologies do not provide spare or idle units for the rerouted communications; instead, the rerouted communications add to existing communications on the new path. In telecommunication systems, trunks and switching fabric paths are typically engineered to run at upwards of 80% occupancy. Therefore, one path cannot support its own and another path's rerouted communications. Hence, the rerouting typically results either in losses of packetized communications or the dropping of calls. Furthermore, these topologies typically do not facilitate repair of the failed units, and consequently result in degradation of system performance over time. Since most of the self-healing networks rely on the use of small, "2.times.2", nodes that are packaged by the tens or hundreds to a board, switching communications from one node to another commonly does not serve to free a board for removal, replacement, and repair.
Tolerance of faults in line circuits (which interface the switching fabric to telecommunication lines, trunks, or other ports) has commonly been handled through line-circuit duplication and through N+K sparing strategies. The disadvantages of interface duplication are the same as of fabric duplication: expense and inefficiency. N+K sparing avoids duplication, by providing some number K of spare standby units that can be substituted for failed ones of the active N units. But, in the past, it has failed to eliminate the disadvantages of duplication, because N+K sparing has traditionally required the use of an additional separate and dedicated switching network, such as a network of multiplexers, along with extensive additional wiring, to reroute communications from failed units to the spare units. Hence, savings gained by avoidance of line-circuit duplication have been lost through inclusion of the rerouting network.